[Translations: /pl] Polish translation

[lwojcik]

Started translating docs into Polish. More iterations coming soon.

[ErisDS]

This translation PR made a good start, but work on it seems to have stopped.

A translation PR should have:

• A folder with copies of all the docs files, correctly named with the IETF language code
• Every file should have the lang and any permalink settings changed
• Your language added to the _config.yml file
• At least one of the mac, windows or linux installation guides fully translated

Once your PR has these 4 things, and you are ready to get it merged, please leave a comment to tell us that your PR is ready.

At this point, two checks need to happen:

1. Someone from the Ghost team will check there are no obvious issues and give a 👍 if all is OK
2. A second person who speaks your language should also check the translations and give them a 👍 if they are happy

Once you get 2 thumbs up, your PR will get merged shortly.

I am going to close this PR for now as it doesn't seem active, but if you want to bring it up to date please let me know and we can reopen it.